Contact printer for photosensitive material

ABSTRACT

A cabinet for use with an optical system for exposure of either or both sides of a workpiece which is sensitive to light. A transparent work holding frame, in which the workpiece is supported by a sheet of glass and covered by a sheet of plastic, is pivotally mounted on the cabinet. Air is withdrawn from between the glass and the plastic by a vacuum system acting through the pivot bearing structure and the vacuum created holds the workpiece firmly in place. A valve is mounted on the cabinet base for adjustment of the vacuum to preclude tearing or breaking the plastic when different sizes and shapes of workpieces are used. A sliding cam-latch is provided on the openable side of the frame to provide a positive closing between the members supporting the sheet of glass and the sheet of plastic. A safety interlock switch is provided to ensure that the lamp unit cannot be actuated when the work-holding frame is either open or pivoted from its normal position.

[ 1 Feb. 22, 1972 United States Patent Boyer, III

[54] CONTACT PRINTER FOR Primary Examiner-Samuel S. Matthews PHOTOSENSITIVE MATERIAL Assistant ExaminerFred L. Braun Attorney-Smyth, Roston 8L Pavitt [72] Inventor: Barton 11. Boyer, 111, Santa Barbara, Calif.

Varo lnc., Garland, Tex.

Nov. 17, 1969 [73] Assignee: ABSTRACT [22] Filed: A cabinet for use with an optical system for exposure of either or both sides of a workpiece which is sensitive to light. A

211 App1.No.: 877,389

transparent work holding frame, in which the workpiece is supported by a sheet of glass and covered by a sheet of plastic, [52] U.S.Cl.....................................355/85,355/73, 355/93,

is pivotally mounted on the cabinet. Air is withdrawn from between the glass and the plastic by a vacuum system acting Int. 27/04 through the pivot bearing structure and the vacuum created [58] Field ofSearch..................355/85, 91, 93, 94,120,122,

holds the workpiece firmly in place. A valve is mounted on the cabinet base for adjustment of the vacuum to preclude tearing or breaking the plastic when different sizes and shapes of [56] References Cited UNlTED STATES PATENTS workpieces are used. A sliding cam-latch is provided on the openable side of the frame to provide a positive closing between the members supporting the sheet of glass and the mm .l Sd 0 e 0." mi woh M m am r d pnm 3; m .H B mi S m kum m mm t .mtv o m Imo w n t. A w w t. %m& nflhd fes O .1 m m m m s. tf 5 3 M SNNS 3553 "5 33" 3,007,390 11/1961 Foresteret a1.... 3,313,225 4/1967 Mears............ 2,117,051 5/1938 Anderson..............

1 Claim, 10 Drawing Figures PAIENTEDFEB22 I972 SHEET 5 OF 5 4770/? war;

BACKGROUND OF THE INVENTION In general, the present invention relates to machines utilized for exposing preselected portions of a workpiece coated with photosensitive material. More specifically, the invention relates to a cabinet structure upon which an exposure or lamp unit including a light source is mounted.

As an example of one application for the invention, modern technology has given rise to the requirements of miniaturization and reliability in electronic circuitry. These requirements have been fulfilled, in many cases, through the use of printed circuits. In producing such circuits, a nonconductive base material, such as a moldable plastic or Pyrex glass, is covered with a thin metallic coating on the order of 0.00l-0.002 inch thick. The metal, such as silver, copper, or aluminum, is usually either chemically deposited on the base or bonded to it in a foil layer, and it is then covered with a coating of photosensitive protective material, in the nature of a photoengravers resist," forming what will hereafter be referred to as a workpiece. Any suitable photosensitive substance, whether wet or dry, having the characteristics of such a resist may be used.

Many of such resists, when exposed to light, are rendered insoluble in suitable solvents, while the unexposed portions remain soluble and are easily removed. While some resists are available which act exactly contrary to this, and could be used with the present invention, the type of resist described will accurately illustrate one problem solved by the present invention.

A photographic transparency may be prepared by drawing the desired circuit in black ink on white paper, exposing the paper to a sensitized film, and developing and fixing the film. The negative is then placed over the workpiece-on top of the resist-and suitable light is passed through the negative to expose or fix the resist. This causes the outline of the circuit to be photographically delineated on the resist.

The workpiece is then washed and the unexposed portion of the resist, which is soluble in a suitable solvent, is removed. The circuit which is to be produced remains covered by the coating of the fixed resist which has become insoluble by exposure to the light. Suitable selection of the resist and an etching solution allows all metal, not covered by the fixed resist, to be removed from the base when it is dipped or washed in the etching solution.

The structure then remaining comprises the nonconductive base having a metal coating, in the shape of the desired printed circuit, which is covered by the fixed resist. If desired,

' another solution may be used to remove the remaining resist.

As the requirements for miniaturization and reliability have become more and more exacting, it has proven to be desirable and necessary to produce workpieces having products of the photosensitive material exposure process on both sides thereof. It has often been necessary to achieve this result by forming two such workpieces which are then fastened together in a back-to-back relationship or by exposing first one side and then the other of a single workpiece.

Machines for exposing the photosensitive material or resist on one side of the workpiece generally have a nontransparent support portion, a flexible plastic cover, a system for forming a vacuum from between the support and the cover. The versatility of a machine in this type is determined by the sizes and shapes of the workpieces which may be placed between the frame elements since a flexible plastic sheet cannot be expected to cover all potential workpieces with the same amount of vacuum, without forming a crease or tear in the plastic. Further, if a workpiece is thick enough to obstruct complete closure of the frame, the vacuum means will fail to withdraw the air from between the frame elements, thereby preventing the transparencies from being firmly held against the work- 'piece. If the machine operator should press against the upper frame element in an attempt to fully close the frame, it is quite likely that he will damage the plastic and/or frame. Additionally, since the exposure unit passes close to the frame to provide maximum light on the workpiece, it is imperative that the frame be fully closed to avoid damage to the unit and the frame.

SUMMARY OF THE INVENTION The cabinet member of this invention basically comprises a frame member upon which all of the structural elements of a light exposure printer can be mounted. The cabinet is adaptable for use with any lamp unit capable of exposing a photosensitive material on either side of the workpiece. on both sides simultaneously. An example of such a unit which may be used to expose either one or two sides of a workpiece has been illustrated in the copending US. application Ser. No. 877,160 of Dexter, filed Nov. 17, 1969 and assigned to the assignee hereof.

In the present invention, an exposure frame is pivotably mounted on the cabinet base in close relationship to the exposure unit. The lower frame element carries a sheet of transparent glass upon the upper surface of which the workpiece and transparencies are supported. An upper frame element is fastened to the lower element by a hinge and carries a sheet of flexible plastic, such as MYLAR, which covers the workpiece. When the workpiece and negatives are put into the frame and it is closed, the operator may pivot the closed frame to a position in which he can visually check the work registration.

In order to ensure complete closure of the frame, a sliding cam-latch is provided, through which the machine operator exerts a closing force in a direction which is parallel to the frame pivot axis rather than tangent to its pivotal arc of rotation.

A vacuum is formed between the frame elements by a tubular structure which passes through the bearings about which the frame pivots, enters the lower element of the frame, and provides a passage through which the air from between the elements is withdrawn when the frame is closed and sealed.

A vacuum adjustment valve is provided which may be easily controlled by the machine operator so that, when a thick workpiece or one having sharp corners is inserted in the frame, the vacuum can be reduced to prevent the plastic sheet in the upper element from becoming creased or torn by the workpiece as it is drawn against it. This is accomplished by means of a spring-biased valve element which is positioned at the junction of the main vacuum line and the lines leading to the frame pivot bearings to cause simultaneous and equal adjustment in all of the lines through which the vacuum is drawn.

A safety interlock switch is mounted on the base for actuation by a magnet mounted on the upper frame element. Unless the upper frame element is in its normal machine operation position, i.e., the frame is closed and not pivoted, the magnet will not actuate the interlock switch and the exposure unit cannot be deliberately or inadvertently driven.

Control means are provided on the cabinet which allow the operator to adjust the exposure unit travel speed as well as the travel distance. Therefore, when a small workpiece is to be exposed, it is unnecessary for the exposure unit to completely traverse the frame; it can be allowed to cross over the workpiece and then return to the home position.

Due to innovations such as these, a machine embodying the concepts of the present invention provide both safety and convenience. The sliding cam-latch produces a positive closure of the frame which ensures the proper formation of a vacuum between the frame elements. If the workpiece should be too thick, however, the failure of the cam follower to enter the cam area of the sliding latch will act as a clear signal to the machine operator and prevent damage to the plastic material. Since the latch is actuated by a force which is parallel to the pivot axis of the frame, rather than tangent to its arc of rotation, the operator can positively close the frame without causing it to pivot, which could upset work registration or damage other machine structure. The formation of the vacuum between the frame elements by structure which passes through the frame pivot points allows the frame to be freely pivoted without necessitating bulky, cumbersome hose structure. Since the vacuum can be adjusted, the degree of contraction of the plastic sheet toward the glass can be controlled so that, when a thick or. sharp-edged workpiece is placed between them, the plastic willnot become creased or torn. On the other hand, when a relatively thin workpiece is placed within the frame, a high degreejof vacuum' can be created to ensure the immovability of the transparencies and the workpiece. The provision of the safety interlock switch will keep the operator from injuring himself or damaging the machine by operating the lampunit before the frame isproperly closed and pivoted to its normal operating position. Since the operator can adjust the lamp'unit travel distance, when a small workpiece is placed withinjthe frame the lamp unit can be driven across the workpiece and then returned to its home position, saving the time of the operator and the electrical power which would be wasted if the unit had to traverse the entire length of the frame before the frame could be reopened and the workpiece removed.

Other advantages and embodiments of this invention will become obvious to those skilled in the art through reference to the detailed description and accompanying drawings which illustrate what is presently considered to be a preferred embodiment of the best mode contemplatedfor utilizing the novel principle set forth in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a photosensitive material exposure printer utilizing the present invention;

"FIG. 2 is another perspective view of the same printer as viewed from the right rear corner of the illustration in FIG. 1;

FIG. 3 is a partial perspective view of the printer shown in FIG. 1, with partsbroken away for a more detailed illustration;

FIG. 4 is a perspective view of the frame member, removed from the cabinet base; I

FIG. 5 is a section view of the framepiv ot structure, taken along the line V-V of FIG. 4; i i i FIG. 6 is a sectional illustration of the dual exposure unit which may beutilized'with thepresent invention, taken along a line VIVI of FIG. 3; FIG. 7 is an enlarged detailed illustration latch used to positively close the frame;

FIG. 8 is a partially exploded, perspective view of the vacuum adjustment valve used in the present invention;

FIG. 9 is an enlarged sectional view-of the valve and valve seat arrangement of the vacuum adjustment valve;

FIG. 10 is a schematic illustration-of the electrical circuitry used in the cabinet of the present invention for control of the exposure unit and other systems.

of the sliding cam- DETAILED DESCRIPTION As shown in FIG. 1, a cabinet 11 supports an exposure unit 13 of any known type, such as that illustrated in the previously cited U.S. application Ser. No. 877,160 of Dexter, for movement across a work-holding frame 15 when actuated by a set of control elements generally designated at 17. In FIG. 6, the exemplary lamp unit 13 is illustrated as comprising a pair of light sources 19 mounted within reflectors 21 which are suitable fastened together, in a manner to be described later, for movement across the frame 15 in which a workpiece and one or more transparencies 23 are suitably fixed.

In FIG. 2, a pair of vacuum hoses 25 have been illustrated which serve to draw cooling air through the reflectors 21 to remove heat generated by the lamps 19. The air is moved by anexhaust pump 27 which causes it to be discharged to the at- 1 mosphere.

rear end of unit 13 for movement along a track 35. A similar bearing (not shown) on the front end plate 29 supports the front end of unit 13 for movement along a track 37. Any suitable means may be utilized to drive the exposure unit 13 from one end of the cabinet 11 to the other. As illustrated, however, an actuator 39 of the ROHLIX type is mounted on a shaft 41 which extends from one end of the cabinet to the other. The actuator is suitably fixed to the housing of the lower lamp 19 so as to move the exposure unit in the axial direction of the shaft as the shaft is rotated by any suitable source such as motor 43 and a pulley system 45. The bearings 33 on the end plates 29 and 31, acting against the tracks 35 and 37, prohibit the lamp unit 13 from revolving about the axis of the shaft 41.

The front end plate 29 also supports a plurality of switch actuators 47, 49, and 51. Actuator 47 operates a reverse limit switch 53 when the exposure unit 13 reaches its maximum travel toward the left, as shown in FIGS. 1 and 3. thereby reversing the motor 43 to drive the unit back to the home, or stop position illustrated in FIG. 1. When the home position is reached, actuator 49 operates a home switch 55, which causes motor 43 to stop all rotation and actuator 51 operates a lamp dimming switch 57 which serves to reduce the power delivered to lamps 19 to preserve the lamps when the unit is not being used, as well as to conserve electrical power. If desired, a home position could be provided at each end of the frame and the circuitry for switch 53 could be modified so as to cause it to be a home switch also and a second lamp dimming switch would be stacked alongside.

Now with reference to FIG. 4, the frame member 15 is shown to be comprised of a lower element 59, which supports a transparent glass sheet 61, and an upper frame element 63, which supports a flexible plastic sheet 65, of any suitable type, such as MYLAR. The upper frame element 63 is pivotally attached to the lower frame element 59 by means of a hinge 67. When the upper frame element is raised, it is held in place by a pair of counterbalancing arms 69 which are biased along rods 71, by springs 73.

The entire frame assembly 15 is pivotally mounted on=a pair of coaxially positioned bearings 75 through which a pair of vacuum tubes 77 pass, as shown in FIG. 5. Each vacuum tube is threaded into a bore 79 in the lower frame element 59 for coaxial communication with a passage 81 therein. In turn, the passage 81 communicates with a groove 83 formed in the upper, internal periphery of the lower frame element. As shown, the glass sheet 61 is seated in the groove 83 and is positioned so as to provide a small gap between the inner end of passage 81 and the glass so that air can be withdrawn from the space 85 between the glass and the plastic 65. A seal element 87, having an extension 89 thereon, is fastened in place by plate 91 fixed to the lower frame element by means of any fastening device such as bolts 93. In the sectional illustration of FIG. 5, the seal extension 89 is shown as ending at the outer edge of groove 83 so that there is no obstruction between the space 85 and the vacuum tube 77. In the areas away from the passage 81, the seal extension 89 extends to the inner edge of plate 91 so that the glass sheet 61 is firmly held against the lower frame element.

As shown in phantom in FIG. 5, when the frame is closed, the seal 87 abuts the upper frame element 63 so as to provide a complete closure about the periphery of space 85.

In order to ensure that the seal 87 is completely operative at the front of the frame, i.e., the side opposite the hinge, an angular member 95 has been mounted near the center of the upper frame element 63 and a sliding cam member 97 has been movably mounted on the lower frame element 59. As shown in FIG. 7, the cam member 97 is fastened to the lower frame element by any suitable means which will allow motion along the direction of the arrow 99. As illustrated, this may take the form of a pair of bolts 111, threadably fitted into bores in the lower frame element so as to slidably cooperate with a pair of longitudinal slots 113 in the cam member. The bolts are positioned within their bores so that the cam member 97 can be moved in either direction of the arrow 99 until the limits of the slots 113 have-been reached. A cam groove having a verticalslot 115 and a camming slot 117 is formed in the camming member intermediate the slots 113. As the upper frame element 63 is brought toward the lower frame element 59, a cam follower pin 119, on the interior of the angular member 95, moves into the vertical slot 115. The operator then moves the camming member 97 toward the left, as shown in the figure, causing the pin 119 to move into the camming slot 117. The camming slot is formed at an angle, relative to horizontal, so that as it moves to the left, it pulls the pin 119 downwardly, thereby forcing the element 63 tightly against the seal 87.

Referring again to FIG. 4, it is seen that a magnet 121 is fixedly mounted on an arm 123 adjacent one edge of the forward portion of the upper frame element 63. When the lower frame element is maintained in the position of FIG. 4 and the upper frame element is brought down to the closed position, magnet 121 will actuate a switch 124 to allow the completion of an electrical power circuit for operation of the machine, as will be described later. When the upper frame element is opened, as shown, the magnet 121 is withdrawn from the vicinity of switch 124, thereby opening the electrical circuit. Further, if the upper frame element is closed but the entire frame is pivoted away from the position of FIG. 4 about the bearings 75, the magnet will be withdrawn from the vicinity of the switch, with similar results.

- to quickly and simply adjust the vacuum created. A hose member 127, from a vacuum source 129 (FIG. 2) is fixed to the valve 125 by any suitable means such as connector 131 for communication with a passage section 133 within the valve. A pair of hoses 135, leading to the vacuum tubes 77, are also in communication with the passage section 133 via connectors 137. Therefore, air being withdrawn from hose 127 by the action of the pump 129 causes a withdrawal of air, through hoses 13S and tubes 77, from the space 85 between the glass and plastic in the frame.

The stem 138 of a valve member 139 is slidably positioned within a bore 141 in the valve body 125 and a head 143 of the valve is biased against a valve seat 145 in the vacuum adjust valve body. The amount of biasing force urging the valve against the seat, exerted by a spring 147, is controlled by an adjustment member 149 threadably fitted within a bore 151 in the valve body. As the adjustment member 149 is turned into the body by the machine operator, the biasing force exerted by spring 147 on the valve 139 will be increased, causing the valve 139 to be more tightly closed and a greater suction to be created in the hoses 135. When the adjustment member is partially withdrawn from the bore 151, the biasing force of spring 147 will be decreased proportionately, allowing greater and greater volumes of air to be drawn through the bore 141, past the seat 145, and into the hose 127. As this occurs, the volume of air withdrawn from the hoses 135, and thus the space 85, is decreased accordingly in an inverse proportion. Therefore,

when a thick or sharp-edged workpiece is placed upon the surface of the glass sheet 61, the vacuum withdrawn from between the glass and the plastic can be properly diminished so as to avoid creasing or tearing the plastic, while still providing a suitable force for maintaining the registration of the transparencies and the workpiece.

A power source 153, shown in FIG. 10, provides power to a main ON-OFF switch 155. When the operator actuates the switch 155, power passes through a pair of lines 157 and 159 to the exhaust pump 27, previously illustrated in FIG. 2. When the pump is turning at nearly full speed, thereby withdrawing approximately the full amount of air through hoses 25, air exhausted from the pump27 through a passage 161 closes a mechanical flapper switch 163 in the line 159 to provide power to the rest of the circuit.

When the operator actuates a vacuum start switch 165, power is provided to the motor 129, to form a vacuum between the frame elements in the manner previously described. When the switch 165 is actuated, a normally open vacuum relief valve 167, which allows atmospheric pressure to enter into the space when switch is open, is closed to prevent outside pressure from entering the space 85.

Electrical power is thus provided to a control circuit 169 for proper distribution to the lamp unit drive motor 43. As shown in the schematic, the reverse limit switch 53 and the home switch 55 serve to control the motor 43 through the circuit 169 to achieve the results previously described.

An interrupt switch 171 also acts through the circuit 169 to control the motor in response to the desires of the machine operator. This switch is actuated by the operator when the lamp unit has passed over a relatively small workpiece and he desires the unit to return to the home position without having to traverse the entire length of the frame.

A cycle switch 173, which includes the previously described interlock switch 124 in its circuit, is actuated by the machine operator when he desires the motor 43 to turn so as to drive the lamp unit across the frame; a speed control unit 175 is also provided to control the rotational speed of the motor 43. Provision of the switch 124 in the circuitry of cycle switch 173 allows the machine operator to position the workpiece and transparency between the frame elements, close the frame, create a vacuum between the elements by operating switch 165 so as to fix the positions of the transparencies and the workpiece, and then tilt the frame away from the operational position so as to check the registration of the lower transparency relative to the workpiece. Since switch 124 is opened during the tilting of the frame, actuation of switch 173 will be futile until the frame is lowered.

Referring now to the right-hand portion of the schematic, it will be seen that two identical circuits have been provided for control of the upper and lower lamps 19. Each circuit is provided with an ammeter 177, visible on the front of the cabinet 11 so that the operator may ascertain whether or not the machine is ready to be used. A switch 179 in each of the circuits provides power, when actuated, to a capacitor circuit 181 provided to drive each lamp 19. Each capacitor circuit is controlled by a relay 183 so that, when power is delivered to the relay in a manner to be described, the capacitor circuit delivers full power to its lamp 19. When power is not delivered to relay 183, the capacitor circuits 181 deliver approximately one-half power to lamps 19, thereby prolonging the life of the lamps and reducing the quantity of power drawn from the source 153.

A normally closed temperature switch 163 is positioned within the exhaust passage 161 leading from the vacuum motor 27. The switch is placed in parallel in'line 157 with the lamp dimming switch 57 previously described with reference to FIG. 3.

In operation, the machine operator closes the main switch 155, thereby starting motor 27. When motor 27 is operating at approximately full capacity, switch 163 closes, delivering power to the remainder of the circuit. Since the temperature switch 185 is normally closed, power is delivered to the relays 183. Therefore, when either or both switches 179 are actuated, full power is delivered to the actuated lamps 19 by the capacitor circuits 181 so that they become properly warmed up. As they become warmed up, the heat given off is removed by the air, passed through passage 161, and over switch 185. When a predetermined temperature has been reached, the switch 185 will snap to an open position. The shutting off of power to the relays 183 will cause the capacitor circuits 181 to deliver half power to the lamps 19 until the machine is actuated. When the machine is in the home position, switch 57 is also opened by the actuator 51 so that it does not provide power to the relays 183.

The operator then positions the workpiece and transparencies between the frame, closes it and locks it by means of the sliding cam latch 97. He then actuates the vacuum start switch 165, causing motor 129 to provide a vacuum between the frame elements. If the vacuum frame interlock 124 is suitably actuated by the magnet 121 with the frame in the closed and operational position, when the operator actuates cycle switch 173, motors 43 will operate through the pulley system 45 to rotate the shaft 41, causing the actuator 39 to be driven along the shaft. As the lamp unit '13 moves away from the home position, switch actuator 51 will be withdrawn from contact with the dimming switch 57 which will then snap to the closed position providing power to the relays 183. This will cause full power to again be delivered to the lamps. The operator then actuates the speed control unit 175 to control thelamp unit travel velocity.

When the lamp unit has completely passed over the workpiece, the operator may actuate switch 171, causing motor 43 to reverse and lamp unit 13 will return to the home position. Alternatively, when the lamp unit reaches the far end of the frame, switch actuator 47 will actuate switch 53, causingan automatic reversal of motor 43 with a similar result.

When the lamp unit returns to the home position, switches 55 and 57 will be actuated, stopping the rotation of motor 43 and shutting off power to relays 183 to reduce the lamp power to approximately one-half. The machine is then ready to be recycled with a new workpiece and transparencies.

Thus, the Applicant has provided a new and unobvious improvement and advance in the art of cabinet structures for printers utilized in exposure of photosensitive materials. Obviously, this cabinet structure can be used with any exposure unit regardless of the type of light and wavelength. Many modifications and alterations of the described embodiment will be obvious to those skilled in the art, wherefore, what is claimed as the invention is:

1 claim:

1. An apparatus for supporting a workpiece relative to a lamp unit for exposure of the workpiece to a light comprising:

a support means a work-holding means mounted on said support means comprising a first transparent member providing a surface for supporting a workpiece placed thereon. a second transparent member providing a surface for covering the workpiece placed on said support surface, and means for positively fixing said transparent members to one another to immobilize the workpiece between said supporting and covering surfaces; guide means on said support means adjacent to and extending beyond said work-holding means at at least one end of said support means, a lamp unit movably mounted on said guides means comprising means for providing light from below said first transparent member and above said second transparent member simultaneously to expose both sides of a workpiece therebetween, and means for moving said lamp unit across said support means for exposure of the workpiece; means for moving cooling air past said lamp unit, means for sensing the temperature of such cooling air after it has passed said lamp unit, and means for reducing the light output of said light providing means when said sensing means senses that such cooling air has attained a predetermined temperature and said' lamp unit is beyond said work-holding means at said at least one end of said support means. 

1. An apparatus for supporting a workpiece relative to a lamp unit for exposure of the workpiece to a light comprising: a support means a work-holding means mounted on said support means comprising a first transparent member providing a surface for supporting a workpiece placed thereon, a second transparent member providing a surface for covering the workpiece placed on said support surface, and means for positively fixing said transparent members to one another to immobilize the workpiece between said supporting and covering surfaces; guide means on said support means adjacent to and extending beyond said work-holding means at at least one end of said support means, a lamp unit movably mounted on said guides means comprising means for providing light from below said first transparent member and above said second transparent member simultaneously to expose both sides of a workpiece therebetween, and means for moving said lamp unit across said support means for exposure of the workpiece; means for moving cooling air past said lamp unit, means for sensing the temperature of such cooling air after it has passed said lamp unit, and means for reducing the light output of said light providing means when said sensing means senses that such cooling air has attained a predetermined temperature and said lamp unit is beyond said work-holding means at said at least one end of said support means. 